US5629435A - Hydrogen sulfide gas sensor and precursor compounds for manufacture of same - Google Patents
Hydrogen sulfide gas sensor and precursor compounds for manufacture of same Download PDFInfo
- Publication number
- US5629435A US5629435A US08/401,540 US40154095A US5629435A US 5629435 A US5629435 A US 5629435A US 40154095 A US40154095 A US 40154095A US 5629435 A US5629435 A US 5629435A
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- Prior art keywords
- tungsten
- hydrogen sulfide
- electrodes
- sulfide gas
- compounds
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
- G01N27/02—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance
- G01N27/04—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance
- G01N27/12—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance of a solid body in dependence upon absorption of a fluid; of a solid body in dependence upon reaction with a fluid, for detecting components in the fluid
-
- C—CHEMISTRY; METALLURGY
- C07—ORGANIC CHEMISTRY
- C07F—ACYCLIC, CARBOCYCLIC OR HETEROCYCLIC COMPOUNDS CONTAINING ELEMENTS OTHER THAN CARBON, HYDROGEN, HALOGEN, OXYGEN, NITROGEN, SULFUR, SELENIUM OR TELLURIUM
- C07F11/00—Compounds containing elements of Groups 6 or 16 of the Periodic System
- C07F11/005—Compounds containing elements of Groups 6 or 16 of the Periodic System compounds without a metal-carbon linkage
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
- G01N27/02—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance
- G01N27/04—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance
- G01N27/12—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance of a solid body in dependence upon absorption of a fluid; of a solid body in dependence upon reaction with a fluid, for detecting components in the fluid
- G01N27/125—Composition of the body, e.g. the composition of its sensitive layer
- G01N27/126—Composition of the body, e.g. the composition of its sensitive layer comprising organic polymers
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N33/00—Investigating or analysing materials by specific methods not covered by groups G01N1/00 - G01N31/00
- G01N33/0004—Gaseous mixtures, e.g. polluted air
- G01N33/0009—General constructional details of gas analysers, e.g. portable test equipment
- G01N33/0027—General constructional details of gas analysers, e.g. portable test equipment concerning the detector
- G01N33/0036—Specially adapted to detect a particular component
- G01N33/0044—Specially adapted to detect a particular component for H2S, sulfides
Definitions
- This invention relates to hydrogen sulfide gas sensors.
- it relates to a chemiresistor coating for electrodes used in hydrogen sulfide gas sensors.
- Hydrogen sulfide is a toxic gas which has the ability to temporarily deaden the human sense of smell. Therefore, there is an important benefit in being able to detect the presence of hydrogen sulfide gas in the environment.
- a chemiresistor sensing device generally contemplates the use of a power supply transmitting current through a sensor which contains a semiconductor material, such as a metal oxide.
- the semiconductor material behaves as a chemiresistor.
- a chemical influence can be caused by an ambient gas interacting with the semiconductor material and can be monitored by a change in the resistance or conductance of the material by the use of electrodes which transmit the change in conductance to a monitor or detector means, such as a voltmeter.
- gases such as hydrogen, anhydrous ammonia, hydrazine, propane, butane, methyl alcohol, ethyl alcohol and hydrogen sulfide (H 2 S).
- Chemiresistor sensors which incorporate thin films of tungsten oxide as the sensing material have been known to respond selectively and sensitively to hydrogen sulfide gas.
- the exposure of tungsten oxide to hydrogen sulfide gas results in a decrease in the resistance of the sensing metal oxide.
- a measurement of the decrease in the resistance of the sensing metal oxide can be used to determine the concentration of the hydrogen sulfide gas.
- chemiresistor sensors comprise a resistor layer, such as a heater resistor, an electrical connection to the heater, a support layer, such as an alumina substrate, a conductor layer (often composed of interdigitated electrodes) and a deposited chemical sensing layer most frequently comprised of tungsten oxide (See for example, Jones et el., U.S. Pat. No. 4,822,465).
- the manner in which the tungsten oxide semiconductor material is applied to the electrodes is of particular importance because the microstructure resulting from the method or technique of depositing the tungsten oxide layer can affect both the selectivity and sensitivity of the tungsten oxide layer to hydrogen sulfide gas.
- the sensors described in Willis et al., U.S. Pat. No. 4,197,089, describe hydrogen sulfide gas sensors with improved selectivity to hydrogen sulfide gas, which comprise a chemically formed sensor film of tungsten trioxide produced by decomposing a droplet of ammonium tungstate contained in solution and deposited on the sensor.
- the patent also discloses a physically formed sensor film of tungsten trioxide which is produced by sintering tungsten trioxide in the powder form on the electrode surface.
- One major disadvantage inherent in the above techniques is an inability to manipulate the microstructure of the film formed.
- Depositing powdered tungsten oxide and sintering the powder or by placing a drop of an aqueous solution containing ammonium tungstate over the electrodes followed by thermal decomposition are rather crude methods for the creation of a film on the electrode.
- Uncontrolled microstructure of the film leads to unpredictable sensitivity and selectivity of the sensing film.
- the inability to manipulate the microstructure of the thin film precludes optimizing the sensitivity and selectivity for a given set of conditions.
- the method used to deposit the thin film will dictate the microstructure of the metal oxide film and, since the microstructure of the metal oxide film may determine the selectivity and sensitivity toward the reducing gas of interest, the method used to deposit the sensing film is very important to its sensing abilities.
- the radio frequency sputtering technique inherently introduces varying levels of stress into the thin film which may effect the sensing capability of the thin films. This stress results from the inability of the sputtering technique to deposit the sensing film uniformly over the surface of the electrode. Conformance to irregular substrates is often poor with sputtered films.
- tungsten carboxylates which can be thermally decomposed to form tungsten trioxide (WO 3 ) and a third genus is disclosed which can be thermally decomposed to sodium tungsten oxides.
- the genera are readily soluble in several commonly used aprotic organic solvents, including aromatic and aliphatic solutions, and can be applied to the electrodes contained in a hydrogen sulfide gas sensor using a precise solution casting technique.
- the invention relates to compounds represented by Formula I
- R is alkyl, alkenyl or aralkyl of 2 to 19 carbons.
- the stoichiometry of the compounds is best represented by the empirical formulae shown, but their actual structures can be monomeric, dimeric or polymeric, as is well known in the art for tungsten carboxylates.
- Preferred subgenera include those in which R is alkyl or aralkyl containing 6 to 10 carbons, particularly 1-ethlypentyl, 2-phenylpropyl and 3-phenylpropyl.
- the invention relates to a method for preparing sodium tungsten Oxide by the thermal decomposition of compounds of formula I and to a method for preparing the compounds of formula I by reacting an alkali metal with an excess of a C 3 to C 20 acid to form a carboxylate-salt solution; and reacting the carboxylate-salt solution with a solution containing tungsten (VI) oxychloride in an aromatic solvent to form a sodium tungsten carboxylate salt.
- the invention may also be described as relating to a tungsten salt, soluble in aprotic solvents, prepared by the process consisting essentially of combining tungsten (VI) oxychloride with four equivalents of sodium 2-ethylhexanoate and a large excess of 2-ethylhexanoic acid in toluene and refluxing for 16 hours.
- n is an integer from zero to three, most preferably n is three.
- R is alkyl, alkenyl, or aralkyl of 2 to 19 carbons, preferably R is 1-ethlypentyl.
- this aspect of the invention may also be described as relating to a tungsten compound, soluble in aprotic solvents, prepared by the process consisting essentially of combining tungsten (VI) oxychloride with thirty equivalents of 2-ethylhexanoic acid and heating at 160° C. for 24 hours.
- the invention relates to a method for coating an electrode for use in a hydrogen sulfide sensor comprising the steps of
- a coated electrode, or a plurality of coated interdigitated electrodes can be fabricated using the novel compounds by dissolving the precursor compounds in a solvent to form a precursor solutions.
- the controlled coating of the electrodes is then accomplished by coating the electrode with the precursor solution using a standard solution casting technique of the type commonly employed for spin-casting or spin-coating, dip-casting or spray coating or casting.
- the electrode is then heated by conventional curing means to decompose the tungsten carboxylate precursor which has been deposited thereon by the desired solution casting technique.
- the decomposition of the uniform thin precursor layer results in a controlled uniform thin layer of tungsten oxide. Once the decomposition occurs, the electrode coating is capable of reacting highly sensitively and selectively to hydrogen sulfide gas.
- the invention also encompasses a hydrogen sulfide gas sensor having an electrode coated with a tungsten oxide derived from thermal decomposition of the novel precursor.
- the object of the invention is to improve hydrogen sulfide gas sensors.
- One advantage of the present invention is the ability to coat electrodes used in hydrogen sulfide gas sensors more precisely, and thereby create a more consistent tungsten oxide or sodium tungsten oxide microstructure on the electrode.
- a further advantage of the invention is the improved substrate conformity of the tungsten oxide or sodium tungsten oxide thin film over the interdigitated electrodes. If the electrode surface were to contain small indentations or protrusions, these imperfections could be compensated for by precisely applying the precursor compound using solution casting techniques.
- a still further advantage of the invention is the improved rheology or "wetting ability" of the compound being deposited on the electrode substrate thereby making it useful in thin film spin-casting or coating, dip-casting and spray techniques, collectively referred to as "solution casting techniques".
- a still further advantage of the invention is the ability to uniformly mix a dopant with the precursor compound and apply a uniform mixture of precursor and dopant through the use of a precise solution casting technique.
- a still further advantage of the invention is the ability to reduce the stress of a thin sensing film which results from non-uniform application.
- the stress of thin films deposited by the solution casting technique is significantly less than the stress measured in thin films deposited by use of a radio frequency sputtering technique.
- a still further advantage of the invention is the ability to coat an electrode more efficiently and cost effectively through the use of solution casting techniques.
- a still further advantage of the invention is the ability to more adequately manipulate the microstructure of tungsten oxide thin films used as sensing films on electrodes for the sensing of hydrogen sulfide gas.
- FIG. 1 is a plan view of a chemiresistor sensor according to the present invention.
- FIG. 2 is a schematic diagram showing a thin film of tungsten carboxylate precursor deposited on interdigitated electrodes.
- FIG. 3 is a graphical representation of a typical relationship between hydrogen sulfide concentration and output voltage of a hydrogen sulfide gas sensor of the type shown in FIG. 1.
- inventive compounds are novel tungsten carboxylates having the formulas I, II and III: ##STR2##
- tungsten carboxylates of Formula I like many tungsten compounds, appear to exist as mixed-valence species (e.g. Formula I envisions one W III and one W IV per unit; the formulas shown represent empirical formulas and not structural formulas. Proposed structural representations of compounds in genus I and III are shown below, ##STR3## but applicants do not wish to be restricted to such structures; compounds made by analogous methods and having the empirical formulas I and III are intended to be encompassed within the invention.
- Compounds of Formula II contain tungsten only in the +2 oxidation state.
- tungsten (II) diacetate they are believed more likely to exist as dimers, or as straight-chain polymers of structure IIb than as monomers as shown in structure IIa.
- structure IIa illustrates an important aspect of tungsten carboxylates: namely, that the tungsten-oxygen bonding cannot be represented as strictly a single bond between one tungsten and two oxygens. Rather, as a result of delocalization of electrons through both oxygens of the carboxyl group, each tungsten is surrounded by four equivalent oxygens.
- Tungsten carboxylates are exemplified wherein R is an aliphatic hydrocarbon, such as C 7 H 15 , but R can be any hydrocarbon chain, so long as the overall solubility and theological properties of the tungsten carboxylate in aliphatic or aromatic hydrocarbon solvents are not significantly changed.
- R is C 6 to C 10 optimize the balance among solubility, rheology and reactivity of the starting acid for forming the tungsten carboxylates.
- the novel mixed valence tungsten (III) and (IV) carboxylates take the form of blue glassy solids and are sensitive to air and moisture.
- the tungsten (II) carboxylates of Formula II are dark green oils or glasses and are also moisture sensitive.
- the compounds of Formula II decompose in the presence of moisture according to the reaction: ##STR5##
- solubility of the compounds in aliphatic and aromatic hydrocarbons makes them useful in solution casting techniques, such as spin-casting, dip-casting and spray-casting.
- inventive tungsten carboxylates may be synthesized according to the following reactions: ##STR6##
- the method of preparing the compounds of Formula I comprises the steps of reacting an alkali metal with an excess of an organic acid to form a carboxylate salt solution; reacting the carboxylate salt solution with a solution containing tungsten (VI) oxychloride in an aromatic solvent in an inert atmosphere to form a reaction mixture; refluxing the reaction mixture to form a sodium tungsten (III & IV) carboxylate, and extracting the sodium tungsten (III & IV) carboxylate from the refluxed mixture.
- Suitable aromatic solvents include toluene and benzene.
- Suitable alkali metals include sodium, potassium and lithium.
- Particularly suitable organic acids include 2-ethylhexanoic acid and 4-phenylbutyric acid or 3-phenylbutyric acid.
- Compounds of generic Formula II are prepared in an inert atmosphere by heating tungsten hexacarbonyl with a large excess of the appropriate carboxylic. acid at reflux, for acids with boiling points below 200° C., or at 200° C., for those boiling higher. The heating is maintained until all the tungsten hexacarbonyl is consumed. The solution is filtered and the excess acid is distilled off under reduced pressure.
- the adapter to the Schlenk flask was purged with nitrogen gas before opening the system to a connected bubbler.
- the 2-ethylhexanoic acid-salt solution was added to the tungsten (IV) oxychloride solution while stirring at room temperature. Under a purge of nitrogen gas, a condenser was connected to the Schlenk flask.
- the reaction mixture was refluxed using an oil bath heated at 125° C. After 16 hours, the solution was cooled to room temperature under a purge of nitrogen.
- the toluene was removed by vacuum distillation. To remove the excess 2-ethylhexanoic acid, a dynamic vacuum was used while heating at 110° C. with an oil bath.
- the glassy blue product was extracted from the sodium chloride in the refluxed mixture with pentane.
- the valve on the reaction flask was closed to prevent exposure to air.
- the reaction flask was taken into the dry box and the solution was filtered through a 0.45 ⁇ cellulose nitrate filter. Using pentane, the remaining material was rinsed out of the flask. A dark blue solid was collected on the filter medium.
- the filtered solution was transferred to a 250 mL one neck flask. The pentane was removed in vacuo and the excess acid was distilled off at 120° C. under vacuum.
- the tungsten carboxylates are ideal precursors for providing a tungsten oxide thin film over electrodes used in hydrogen sulfide gas sensors. It is well known that tungsten oxide is an ideal film for coating of electrodes in hydrogen sulfide gas sensors, because tungsten oxide films have shown good selectivity and sensitivity to hydrogen sulfide gas.
- the resistance to a current passed through the chemiresistor comprised of a tungsten oxide film coating on electrodes decreases when hydrogen sulfide is in the ambient gas. The decrease in resistance is believed to be caused by an exchange/reduction between O -2 and S -2 with the production of WS 2 , which has a greater conductivity than WO 3 .
- the resulting exchange between O -2 and S -2 can be measured by an increase in voltage at a detector device. This is accomplished by having the sensor connected to a standard operational amplifier circuit incorporating the detector device. The decrease in resistance translates into an increase in voltage which is relative to the concentration of the hydrogen sulfide gas.
- FIG. 3 shows the relationship between the concentration of the hydrogen sulfide gas and the increase in voltage of the sensing device caused by the decreased resistance of the chemiresistor.
- the present invention provides an improved film coating of tungsten oxide on the electrodes used in hydrogen sulfide gas sensors.
- the film of the novel tungsten carboxylate precursor of the present invention is applied or deposited on the electrodes, preferably arranged in an interdigitated configuration, by a known solution casting technique. While it is not possible to solution cast tungsten oxide (because it is insoluble in solvents typically used in solution casting processes), the inventive tungsten carboxylate precursors can be applied to electrodes, including interdigitated electrodes supported on an inert substrate by solution casting techniques. This is possible because the novel carboxylates are soluble in the solvents used in solution casting techniques and have the necessary rheology and surface wetting properties.
- the resulting thin films from II and III decompose to tungsten oxide when heated to above approximately 350° C. by conventional curing methods; the films from I decompose to sodium tungsten oxide.
- a general procedure for coating a substrate is provided by the following example:
- FIG. 1 shows a sensor 22 of the present invention having a substrate support layer 10 made from inert materials, such as quartz and containing or having mounted thereon conductors of a conductor layer 11, 12, 13, 14.
- the conductors 11, 12, 13 and 14 are made from conducting material, such as gold or palladium. Electrical current can be passed from a standard power supply via a conducting wire or other means through the conducting layer 11.
- the conducting layer 11 is in contact with an adjacent resistor or heater layer 15.
- the resistor layer 15 generates heat from the conducted current.
- the current is then passed from the resistor layer 15 to the conductor 14 and via a standard conducting means back to a power supply.
- Electrode layer 16 On the upper side of the resistor layer 15, there is a silicon-oxide based dielectric layer 16, upon which there is mounted an electrode layer comprising electrodes 17 and 17'.
- a sensing film 18 according to the instant invention is deposited over the electrodes 17 and 17'.
- the dielectric layer 16 functions in the sensor 22 to shield the resistor layer 15 from reacting directly with the sensing film 18.
- the resistor layer 15 heats the coated electrodes 17 and 17' to improve sensitivity and selectivity of the sensing film 18, as is commonly done in gas sensor technology.
- Electrode 17 Electrical current is also passed from a power supply through a conductor 12, to the electrode 17.
- the current transfers to electrode 17' and is passed through the conductor layer 13 which is connected to a standard operational amplifier circuit with a detector means, of the type known in the art.
- the electrode layer 17 and 17' is preferably arranged as interdigitated electrodes which have been coated with the sensing film 18, using a solution casting technique.
- the sensing film 18, is a tungsten oxide or sodium tungsten oxide thin film formed from thermally decomposing the novel tungsten carboxylate compounds.
- the sensing film 18, selectively reacts with hydrogen sulfide gas in the ambient atmosphere to cause an increase in the conductance of a current passed through the electrodes 17 and 17'.
- FIG. 2 shows a schematic representation of the thin sensing film 18, deposited on an interdigitated array of conducting electrodes 17 and 17'.
- FIG. 3 is a graphical representation of a typical relationship between hydrogen sulfide concentration and output voltage of a hydrogen sulfide gas sensor 22 of the type shown in FIG. 1 when coated according to the general procedure with the compound of example 1 and heated at 500° C. The trend shows that there is an increase in output voltage with an increase in hydrogen sulfide gas concentration.
- tungsten oxide reacts with hydrogen sulfide gas to form tungsten sulfide.
- Sensors fabricated according to the present invention have improved substrate conformity, a more uniform doping ability, less potential stress in the films and are more conveniently fabricated than those made by known methods.
Abstract
Description
Na[OW(OOCR).sub.2 ].sub.2 I
ClO.sub.3 W.sub.3 (OOCR).sub.2 III
TABLE 1 ______________________________________ Compound Film Deposit Heating of example # Conditions Conditions (°C.) Nature of film ______________________________________ 1 17 mg/50 μl 500°/hot plate sodium tungstate 2 17 mg/50 μl 520°/hot plate hexagonal phase WO.sub.3 2 40 mg/100 μl 300°/oven amorphous WO.sub.3 2 40 mg/100 μl 500°/oven cubic phase WO.sub.3 containing small amount of triclinic 2 40 mg/100 μl 300°/hot plate cubic phase WO.sub.3 containing small amount of triclinic 2 40 mg/100 μl 500°/hot plate cubic phase WO.sub.3 containing small amount of triclinic 3 9 mg/100 μl 300°/oven amorphous WO.sub.3 3 9 mg/100 μl 500°/oven partially crystal- line cubic phase WO.sub.3 3 9 mg/100 μl 300°/hot plate amorphous WO.sub.3 3 9 mg/100 μl 500°/hot plate partially crystal- line triclinic phase WO.sub.3 ______________________________________
WS.sub.2 +7/2 O.sub.2 →WO.sub.3 +2SO.sub.2
Claims (7)
Na[OW(OOCR).sub.2 ].sub.2
Na[OW(OOCR).sub.2 ].sub.2
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US08/401,540 US5629435A (en) | 1991-03-29 | 1995-03-10 | Hydrogen sulfide gas sensor and precursor compounds for manufacture of same |
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US67772991A | 1991-03-29 | 1991-03-29 | |
US07/934,920 US5321146A (en) | 1991-03-29 | 1992-08-25 | Hydrogen sulfide gas sensor and precursor compounds for manufacture of same |
US08/200,479 US5433971A (en) | 1991-03-29 | 1994-02-23 | Hydrogen sulfide gas sensor and precursor compounds for manufacture of same |
US08/401,540 US5629435A (en) | 1991-03-29 | 1995-03-10 | Hydrogen sulfide gas sensor and precursor compounds for manufacture of same |
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Cited By (3)
Publication number | Priority date | Publication date | Assignee | Title |
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US20090280593A1 (en) * | 2008-05-07 | 2009-11-12 | Honeywell International Inc. | Matrix nanocomposite sensing film for saw/baw based hydrogen sulphide sensor and method for making same |
US11143641B1 (en) * | 2021-04-05 | 2021-10-12 | Vivante Health, Inc. | Gas sensor calibration method |
US11331019B2 (en) | 2017-08-07 | 2022-05-17 | The Research Foundation For The State University Of New York | Nanoparticle sensor having a nanofibrous membrane scaffold |
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DE60229744D1 (en) * | 2002-09-11 | 2008-12-18 | Microchemical Systems S A | Chemical gas sensor and related manufacturing process |
US7438079B2 (en) * | 2005-02-04 | 2008-10-21 | Air Products And Chemicals, Inc. | In-line gas purity monitoring and control system |
US11353419B2 (en) * | 2018-05-27 | 2022-06-07 | Tao Treasures, Llc | Compositions and methods for gas sample analysis |
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